How to repair damaged flooring timbers

Left uncorrected, damage to flooring timbers is potentially dangerous. Every effort must be made to remedy the problem before the trouble reaches unmanageable proportions and you are faced with huge repair bills.

Although a seemingly major task, the repair and renovation of damaged joists and flooring is well within the capabilities of any competent do-it-yourselfer. It is not the sort of job you have to do often, but may encounter if you decide to renovate an old or neglected property.

Suspended floors

Most houses feature some form of suspended wooden floor.

For the downstairs floor, floorboards are laid across loadbearing timbers called joists. These in turn are laid at right-angles on low, supporting brickwork of one brick width in the form of piers, or in some houses, sleeper walls.

The sleeper walls are laid at maximum intervals of 1.8m across the length or width of a room, on a supporting base which consists of at least 100mm of compacted hardcore, levelled with a layer of sand called blinding, and topped with a 100mm coat of concrete which is usually finished with a thin layer of levelling screed. This solid base covers the whole floor area between the inner wall foundations, and is usually referred to as the over-site concrete.

The lack of an oversite concrete base and consequent dampness in old houses leads to most of the problems you are likely to encounter with flooring. The problem is aggravated by inefficient underfloor ventilation , and the use of a single course sleeper wall which is very prone to trouble and a bad characteristic of early suspended floor design. Nowadays, three brick courses are normally employed for sleeper walls.

The brickwork of sleeper walls is usually laid in a honeycombed construction, always a little away from main walling, so that underfloor ventilation is improved. A special variation, a fender wall, supports the hearth of a fireplace as well as joist ends. The thickness of this varies between 102mm and 215mm according to the wall height and load which has to be supported, but is otherwise built like any other brick walling.

The honeycombed construction does not have to be precise or to any specific pattern. The aperture can be achieved simply by leaving out a brick every so often when making the wall, but a better method is to leave a third-brick gap between each brick. This ensures a plentiful quantity of ventilation holes as well as providing a strong construction.

Each sleeper wall is topped by a stout timber member which acts as a bearing for the joists. Called a wall-plate, this strip of wood is firmly bedded on lime mortar, nowadays above a bitumen felt DPC so that rising damp does not spread to woodwork above. The depth of the mortar bed can be adjusted to level the timber.

The wall-plate acts as the fixing point for the joist ends as well as uniformly distributing floor loads along the length of the sleeper wall.

There is unlikely to be a DPC on sleeper walls of very old properties and this, combined with the lack of oversite concrete, often results in rotting floorboards and joists. In replacing these, you must be sure to incorporate a DPC—in the form of a strip of bitumen felt—underneath each wall-plate to be protected.

Joists are attached to the wall-plate by nails angled through their sides. Housing joints can be made in the wall-plate to level the joist if the sleeper wall or bedding mortar has caused a slope in the wall-plate. But under no circumstances should pieces of slate, wood or newspaper be used for levelling a joist. If necessary, use a deeper joist in conjunction with housing joints where the joist depth has to be increased at one end.

You may encounter more complicated forms of jointing such as notching and cogging but the resulting joints are rarely used in modern building work.

Although hardwood such as oak can be used—quite probable in old buildings—high cost and restricted availability makes the use of softwood more or less a necessity today. Redwood is considered the most satisfactory wood for both wall-plates and joists.

The maximum clear span for a 50mm

X 100mm joist is about 2m when the joists are at 400mm centres and this is suitable for supporting floorboarding of up to 25mm thickness, in anticipation of ‘normal’ floor loadings and grades of timber. For spans greater than 2m, deeper joists have to be used —but in practice all you have to do is match the size of existing joists when it comes to getting a replacement. You can, of course, use unplaned timber.

Floorboards or flooring-grade chipboard is fixed to the joists to complete the flooring. The long edges of chipboard are supported on the joists, but short battens— noggins—must be nailed between the joists to support the short edges between boards.

All supporting timber must be treated with a suitable insecticidal and fungicidal preservative before construction or replacement begins.

Ground floors in Canadian houses are often above a basement. Instead of sleeper walls, the joists are usually carried on beams which are in turn supported on columns. Both beams and columns may be wood or steel. The ends of the joists around the perimeter of the house are often supported on wooden wall-plates which are bolted to the foundation walls.

Joist depth depends on span and the grade and species of lumber used: 50mm x 200mm, for example, can span between three and four metres.

Upstairs floors

Problems with upstairs floors are much less common.

Joists are supported- in one of a number of different ways. The joist ends can be supported by recesses in the inner main walling but the use of metal hangers of various types is now much more common.

In old houses, joists actually built into the wall are a prime target for rot caused by rising damp in the wall. In replacing these joists, provide a larger cavity—if necessary by removing additional brickwork—so that the new joist end receives at least some ventilation. Use a metal wall-plate in preference to a DPC resting on a mortar bedding. Rising damp is much less of a problem in modern houses with cavity walls and effective DPC so built-in joists are perfectly feasible.

The use of corbels, protruding ledges of brick or stone, and offsets, stepped ledges formed by decreasing the wall width as each floor is reached are other traditional forms of upstairs flooring which you may encounter in old houses. If a DPC is not included in the wall, make sure to add this below a wooden wall-plate when fitting the replacement timber.

Modern construction methods tend to rely ever increasingly on the use of galvanized metal hangers. These are available in all joist sizes, and can be used in a variety of ways. Normally they are mortared into the brickwork on the inner main walls or hung over internal party walls to connect adjacent joists.

One type—the corbel bracket— simulates the function of a corbel. Built into the walls at joist space intervals, they support short lengths of wall-plate timber which in turn support the joists.

You are highly unlikely to encounter problems with metal hangers, but they are useful in repair work as alternatives to the more complicated traditional supports.

Also, in old, damp houses, temporarily inserting hangers into brickwork above or below the existing joist supports enables you to raise or lower the floor and make good damaged corbels, cavities and offsets.

Upstairs joists can be supported only by their ends and unless a supporting internal party wall is featured, they may extend the width of the room. The joists are therefore relatively large, and often more closely spaced than those used for the ground floor.

Another difference is that bracing is used to minimize the tendency of long lengths of wood to bow and of deep joists to twist or tilt. Two types of bracing are commonly used, at spacings not exceeding 1.8m.

The simplest is solid strutting , though this must be used in conjunction with a bracing rod otherwise it is completely ineffective. Short lengths of floorboard are nailed in line between the joists, in effect to maintain an even and constant joist separation when the nearby iron rod is screwed tight. This method is now very rarely used, but you may encounter it in repairs to older houses.

Herringbone strutting is much the better of the two methods and consists of pairs of inclined pieces of timber fitted as tightly as possible between the joists. Timber 50mm X 32mm or 50mm is used for the job and is simply nailed into place once a suitable sloping face has been cut.

Another feature of upstairs floors is trimming. This describes what happens when a joist cannot, for some reason or other, be supported at both ends by walls of the building—normally the problem is a fireplace or stairwell. In this case, support must be provided by imposing on any nearby joists which do not have to terminate short of their wall support. These then become known as trimming joists. A fixing bracket is used to carry a trimmer joist which is used to support trimmed joists. To carry the extra load, these joists are usually thicker, or may consist of two joists nailed face-to-face.

In Canadian houses, the joist ends of upper floors are usually supported on the top plate of the wall framing of the floor below. Intermediate support is on the top plate of partition walls, or on beams as for ground floors. Herringbone strutting is called ‘diagonal bridging’; solid strutting is ‘blocking’.

Trouble spots

If you suspect trouble in your flooring, raise all the floor covering and start by prodding boards and skirting with a bradawl to check for exterior signs of wet and dry rot. Proceed by raising a floorboard here and there to check the state of joists and sleeper walls.

However, to most of us the first sign of trouble is a broken or rotten floorboard. Start by removing this, along with all surrounding flooring which seems to be similarly affected.


Before going any further, identify the cause of the problem. In some instances you will be able to do no more than replace old with new. If rot seems to be the trouble at least you know what needs to be done, but professional advice should be sought so that the problem can be properly cured before you proceed further.

Sleeper walls are usually fairly trouble-free, but dampness or ground subsidence can occasionally cause them to disintegrate.

In this case, there is no alternative but to take up the floorboards, temporarily support or remove the joists and rebuild the affected sleeper wall.

The floor above a basement in Canadian houses sometimes sags. This can often be corrected without having to rebuild the whole floor by jacking it back into place, taking very great care. Use a short house jack, extending its height with a wooden post and spreading the load well, both on the basement floor and on the ceiling of the sagging floor above. Screw up the jack no more than a quarter of a turn a day until the sag has gone. Then support the joist permanently with beams and columns, properly sized and fixed.

The lack of an oversite concrete base encourages the presence of damp and if ventilation is in any way restricted, moisture has little chance to clear and rot sets in. The reverse can happen too: dry rot can readily form in an enclosed space which remains unventilated.

The most common cause of inadequate ventilation is a blocked-up airbrick. Particular care must be taken not to obscure airbricks without providing alternative ventilation when adding an outside extension or patio —a frequent oversight. And clearing airbricks of dust and accumulated rubbish must be included in your list of routine household maintenance tasks if long-term flooring problems are to be avoided.

Take additional precautions to see that surface rainwater drains away from the building and, in the case of doorways, well clear of the floor woodwork. As an outside doorway is subjected to higher than average exposure, the first signs of trouble are likely to be in this area.

Repairing a damaged floor

Start by pulling up the floorboards that obviously must be discarded and burnt, then check the extent of hidden damage. Bear in mind that you will have to remove a certain amount of the sound wood around any rot that // these interfere may be present: a margin of about 0.6m is normally sufficient for wet rot and 1m for dry rot.-

If, for example, a wall-plate contains an area of rot, carefully check the surrounding joists and floorboard-ing to see if the damage has spread to these too; replace everything if there is the slightest sign of contamination.

A joist can also be removed in sections. Cuts should be made at the recommended distance from the rot affected area , to coincide with wall-plates. You will need to remove a slightly larger area of floorboarding in order to gain sufficient access.

Separate the joists from the supporting wall-plate by sliding a hacksaw beneath the two in order to cut the fixing nails—unless, of course, the nail head is exposed and can be re- moved with a claw hammer.

Wall-plates are best removed in short sections but as you do so, carefully support the remaining joists with bricks. You may find that the replacement wall-plate has to be fitted in two sections in order to give you sufficient manoeuvrability. All the new timbers must be treated with a suitable preservative to guard against insect and fungal attack.

To level a wall-plate, use a wooden wedge to prop up one end and infill the nearby gap with suitable mortar. Allow this to dry, remove the mortar and complete the filling. Do not use any form of loose packing.

With upstairs floor repairs, it is difficult to avoid damaging the ceiling below. Plasterboard can be pushed gently downwards to disengage from the joist underside enough for you to slide in a hacksaw blade and sever the fixing pins. You will need an assistant with a ‘lazy man’ to avoid damaging the loosened sheet if this is to be saved.

There is no simple and effective way of replacing just portions of the upstairs joists, so you have to resign yourself to the fact that new, full-length joists must be fitted wherever rot is detected in the old ones. Make a point of replacing strutting in the vicinity of the damaged area.

Where joists are set into the walls and obviously suffering from damp, use shorter new joists and support them on hangers. Cut the slots for the hangers with a hammer and bolster then use a 1:3 mortar mix to set them in place, propped at the correct height. Do not fit the joists until you are sure that the mortar has completely set.

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